Either 2003 or 2004.
USA won team gold for WAG in 2003 and the MAG won team silver, nearly beating China for gold. Both were the best results at a Worlds for either team (MAG won silver in 2001 as well). Additionally Paul Hamm won the AA and Patterson got silver. US also had three individual champions, Memmel (UB), Vise (UB), and Paul Hamm (FX).
2004 was also a strong year with both teams winning silver, Hamm and Patterson took AA gold, Patterson, Humphrey, and Hatch took silvers and Kupets got a bronze, with Hamm also taking HB silver.
If you are just looking at WAG, then 2005, was by far and away the best year for USAG. They went 1 and 2 in the AA, UB, BB, FX. Sacramone took home bronze on vault, and .006 away from silver. The ladies took 8 of the possible 10 gold and silver medals. They also won 9 out of 10 medal chances (2 per country X 5 events. Had there been a team competition, there is little doubt that Bieger, Sacramone, Liukin, Memmel, Kassi Price (UB/FX) and Annie DiLuzio (VT/BB) would have destroyed the field for the team gold.